Suppression of noise in an integrated circuit

ABSTRACT

Sub-circuits of an integrated circuit can act as noise sources on common conductors such as power supply lines and the substrate. Each of these conductors may act as a noise medium capable of transferring noise signals from the noise source to other sub-circuits. One or more feedback circuits are coupled between input and output points on opposite sides of where a circuit to be protected is connected to such a medium, so that a output of the feedback circuit is coupled to the noise medium closer to certain noise sources than the input of the feedback circuit. Preferably, multiple feedback circuits are cross-coupled and have transfer connections so that coupling between the input and outputs of different feedback circuit is at least partially suppressed.

The invention relates to an electronic integrated circuit and more inparticular to the suppression of interference noise in such a circuit.

From U.S. Pat. No. 6,040,728 it is known to reduce noise in anintegrated circuit by means of injection of noise-canceling signals. Inan integrated circuit many different sub-circuits are combined on thesame substrate. Apart from the intended signals, each sub-circuitgenerates undesired signals, termed noise, in the substrate. Thesubstrate acts as a noise medium which permits the noise from onesub-circuit to affect other sub-circuits in undesirable ways. Thus,sub-circuits also act as noise-sources. Noise that reaches analogcircuits from digital circuits, for example, leads to deterioration ofthe signal-to-noise ratio of the analog circuits.

U.S. Pat. No. 6,040,728 teaches how a “quiet” region can be realized inthe substrate of an integrated circuit, where sensitive analog circuitsare present on the substrate (which acts as a noise medium). For thispurpose a plurality of feedback circuits are provided in an array alongthe boundary of the quiet region. Each feedback circuit regulates thevoltage in the substrate at a respective point along the boundary of thequiet region, so that the local voltage equals a “quiet” referencevoltage. To regulate the local voltage, the feedback circuit senses anoise affected signal at the respective point and feeds back currentthat is negatively proportional to the difference between the localvoltage and the reference voltage.

Thus, the noise signal is reduced at least locally and it may beexpected that within the region surrounded by the respective point thenoise is reduced as well. This technique requires a large number offeedback circuits. Even so, noise inside the region is reduced onlypartially when the spatial variation of the effect of the injectedcounter-noise as a function of position away from the point of injectionis not the same as that of the noise. This is the case in particular fornoise generated inside the region.

Amongst others, it is an object of the present invention to improvenoise suppression when the noise to be suppressed has a spatiallyvarying strength, and in particular when the noise source is locatedwithin a region where noise should be suppressed.

Amongst others, it is a further object of the present invention toreduce noise over a larger region using multiple non-local feedbackcircuits, while reducing a tendency towards local feedback effects.

Spatially varying noise from noise sources can be suppressed better byusing non-local feedback, that is, a feedback wherein the noise sourceand an injection point of the feedback are coupled to a noise medium ona first side of the sub-circuit that is to be protected against noiseand an input of the feedback is connected to the noise medium on asecond side of the sub-circuit, the second side being opposite the firstside. Such non-local feedback may be applied for example to suppress theeffect of power supply bounce propagation on a noise medium like a powersupply line in the integrated circuit, or substrate noise propagationacross a digital-analog boundary in substrate of the integrated circuit,by using a feedback circuit with an input and an injection point coupledto the power supply line at positions removed from one another, so thatat least some protected sub-circuits are connected to the power supplyline between the input and the injection point. Thus, the input is in aregion of the medium relatively far away from the injection point andthe noise source. In such a region the spatial variation of the effectof the injection point and the noise source is less strongly dependenton position, or both effects may even depend congruously on position.Therefore cancellation of the noise source and the injection occurs overa large region. Cancellation is not strongly localized, as in the caseof local feedback, because with local feedback the effect of injectionand the effect of the noise source around the input depend on positionin a different way since the input is near the injection point and thenoise source is not.

The use of non-local feedback may have the disadvantage that there is adetrimental effect on the noise from noise sources in regions away fromthe region where noise is suppressed, for example when the noise sourceis opposite the injection point, as seen from the input of the feedbackcircuit. To suppress noise from sources in more than one direction, itwould be desirable to inject different counter-noise at more than onelocation, using different feedback circuits. However, when multiplefeedback circuits are combined with non-local feedback it may be thecase that the point where one feedback circuit senses noise is close tothe point where another feedback loop injects noise and vice versa. Thusan overall feedback loop is created which includes both feedback loops.Such an overall feedback loop suppresses noise locally at an input byinjection of counter-noise close to the input, thus undoing theadvantages of non-local feedback.

In an embodiment the integrated circuit comprises a further noise sourcecoupled to the noise medium and a further non-local feedback circuitwith input and output coupled to the noise medium, the inputs of thenon-local feedback circuit and the further non-local feedback circuitbeing differential inputs, the differential inputs of the non-localfeedback circuit being coupled to the noise medium on mutually oppositesides of the further noise source, the differential inputs of thefurther non-local feedback circuit being coupled to the noise medium onmutually opposite sides of the noise source. It has been found thatnoise is reduced in this way. A tentative explanation is that thisspatial feedback attempts to equalize the feedback signal at its two endpoints and as such attenuates the noise magnitude comprised within theloop. As a result the contribution of the noise from these sources tooverall feedback effects is reduced. This at least partly permits theoperation of multiple non-local feedback circuits with non-localfeedback effect.

In another embodiment the effect of local feedback is suppressed byusing a further non-local feedback circuit, with input and outputcoupled to the noise medium, so that an output of the non-local feedbackcircuit is coupled to the noise medium closer to the input of thefurther non-local feedback circuit than to the input of the non-localfeedback circuit, the output of the non-local feedback circuit and theinput of the further non-local feedback circuit being mutually arrangedso that transfer of output signals from the output of the non-localfeedback circuit to signal components fed back by the further non-localfeedback circuit are at least partially suppressed. Thus, it is ensuredthat noise canceling extends over relatively large regions, by forcingcompensation of the noise in each region by injection relatively remotefrom that region. Suppression masks inputs and outputs from one another.Masking may be realized for example by providing the feedback circuitswith differential inputs coupled to the medium, the feedback circuitsweighing signals from the inputs so that input signals from a nearbyinjection point substantially do not contribute to the feedback signal.As an alternative masking may be realized with differential outputcoupling weighted so as not to affect nearby inputs.

These and other objects and advantageous aspects of an integratedcircuit according to the present invention will be described in moredetail using the following figures.

FIG. 1 shows a circuit with noise compensation

FIG. 2 a,b show noise amplitudes as a function of position along a noisemedium

FIG. 3 shows an embodiment of a circuit with noise compensation

FIG. 4 shows a further embodiment of a circuit with noise compensation

FIG. 1 shows a circuit with a first power supply conductor 10 and asecond power supply conductor 11 between which a number of sub-circuitsis connected (only four sub-circuits 12 a-c shown). A feedback circuitis provided, comprising a feedback inverting-amplifier 15. Feedbackamplifier 15 has an input coupled to an input point 14 a on first powersupply conductor 10 and an output coupled to an output point 18 remotefrom the input point 14 a. Power supply conductors 10, 11 act as mediafor transmission of signals, i.e. the voltages at different points alongthese conductors may be mutually different due to propagation effects,for example due to RC filter-like behavior or propagation delay. Toemphasize the transmission medium like nature of power supply conductors10, 11 symbolic filters 100 (only one labeled) have been shown includedin the power supply conductors.

In operation sub-circuits 12 a-d draw power supply currents which flowbetween power supply conductors 10, 11. The power supply currents causevoltage fluctuations on the power supply conductors 10, 11. Throughthese voltage fluctuations, sub-circuits 12 a-c may exert undesirableeffects upon one another. These voltage fluctuations should be regardedas noise. Thus, power supply conductors 10, 11 act as noise media andsub-circuits 12 a,b act as noise sources in this respect. Feedbackamplifier 15 counteracts the effect of noise. Feedback amplifier 15senses voltage fluctuations at input point 14 a and injects a currentthat generates counteracting voltage fluctuations at output point 18.This generally requires that feedback amplifier 15 has a negative gainfactor.

FIG. 2 a schematically shows a first amplitude 20 of noise fluctuationscaused by a first sub-circuit 12 a as a function of position X alongpower supply line 10. (It will be understood that power supply conductor10 may in fact follow a path with bends in it, in which case Xrepresents distance traveled via the relevant path). FIG. 2 afurthermore shows a second amplitude 22 of counteracting noisefluctuations introduced by feedback amplifier 15 in response to thenoise fluctuations caused by first sub-circuit 12 a (The actualcounteracting noise fluctuations introduced by feedback amplifier 15 arethe sum of the responses to the noise fluctuations due to differentnoise sources such as sub-circuits 12 a-d. It will be understood that inpractice only this sum is observable, not the individual responses todifferent sub-circuits 12 a-d). The X position 24 of output point 18 andthe X position 26 of input point 14 a have been indicated.

Under ideal feedback conditions, fluctuations at the input point 14 a (Xposition 26) due to first sub-circuit 12 a have the same amplitude asfluctuations at that input point 14 a due to the response by feedbackamplifier 15, but are of opposite sign, so that the net noise at inputpoint 14 a (X position 26) is zero. In a relatively wide range of Xpositions around the X position 26 of input point 14 a noise, where theX-positions are remote from both the location of first sub-circuit 12 aand output point 18, the net noise is also low, because noise due tofirst sub-circuit 12 a and counter noise due to feedback amplifier 15have substantially the same X position dependence in this region.

By way of contrast FIG. 2 b shows the situation that would have arisenwhen the output of feedback amplifier 15 would have been coupled topower supply conductor 10 substantially at the location of input point14 a. In this case, the amplitude 28 of counter noise due to feedbackamplifier 15 would generally be at a maximum at the X position 26 of theinput point 18. Consequently, noise due to first sub-circuit 12 a andcounter noise due to feedback amplifier 15 have substantially dissimilarX position dependence around this position. Thus the net noise differsfrom zero only over a relatively narrow region.

Thus, by providing non-local feedback, with at an output point closer tothe noise source 12 a than the input point noise due to remote noisesources is cancelled over a relatively wide region where sub-circuits 12c-d are coupled to power supply conductor 10. A single feedbackamplifier 15 suffices for the purpose of noise canceling when the onlyrelevant noise source is remote in this sense. However, preferably aplurality of at least two feedback amplifiers is used when there areother noise sources.

FIG. 3 shows a circuit according to the present invention wherein twofeedback circuits are provided, each comprising a feedback amplifier 35,37. A first feedback amplifier 35 has differential inputs coupled toinput points 14 a,b on first power supply conductor 10 and an outputcoupled to an output point 18 remote from the input points 14 a,b. Asecond feedback amplifier 37 has differential inputs coupled to inputpoints 16 a,b on first power supply conductor 10 and an output coupledto an output point 19 remote from the input points 16 a,b.

In operation, like amplifier 15 of FIG. 1, feedback amplifiers 35, 37 ofFIG. 3 have the effect of suppressing noise on power supply conductor10. (It will be understood that the word “feedback” is taken to implythat the sign of amplification by feedback amplifiers 35, 37 is so thatan increase in the input signal leads to an output signal thatcounteracts the increase). Feedback amplifiers 35, 37 differentiallysense noise voltage fluctuations on power supply conductor 10 betweentheir respective input points 14 a,b and 16 a,b and introducecounteracting noise at their respective output locations 18 and 19.Noise from noise sources in a broader range of locations is suppressedthan would otherwise be possible with a single feedback amplifier.Differential sensing reduces the effect of localization which may arisewhen the respective output point of one of the amplifiers 35, 37 islocated near the respective input point of the other amplifier 37, 35and vice versa. If feedback amplifiers 35, 37 both would have had singleended inputs (not differential inputs) near the output point of theother amplifier, the two amplifiers would have tended to cooperate as asingle feedback loop, the output of each amplifier 35, 37 serving tosuppress noise locally, each substantially as shown in FIG. 2 b. As aresult, this reduces the range of locations where noise is below a giventhreshold. It has been found that differential sensing reduces thiseffect, providing a broader range in which noise is below the threshold.

FIG. 4 shows a further embodiment of a noise suppression circuitaccording to the present invention. (Here as well it will be understoodthat the word “feedback” is taken to imply that the sign ofamplification by feedback amplifiers 35, 37 is so that an increase inthe input signal leads to an output signal that counteracts theincrease). In FIG. 4, single end gain factor adjustment circuits 40, 42have been added at the inputs of differential amplifiers 35, 37. Inoperation single end gain factor adjustment circuits 40, 42 provide anadjustment of the sensitivity of the differential input of thedifferential amplifiers 35, 37, so that for each amplifier 35, 37 therespective output signal Y (not illustrated) of the amplifier 35, 37depends on the respective voltages Va, Vb (not illustrated) at the inputpoints 14 a,b, 16 a,b according toY=g(Va−αVb)

The factor α is realized by the single end gain factor adjustmentcircuits 40, 42 and chosen so that counter noise introduced at an outputpoint 19 near the input points 14 a,b at which Va and Vb are sensedsubstantially has no effect on the output signal Y. That is, if thetransfer factor of output signals Y from the nearby output point 19 tothe input points 14 a,b that sense Va and Vb are Ha and Hb respectively(not illustrated), the factor α equals Ha/Hb. When the single end gainfactor adjustment circuits 40, 42 are applied to the input points 14 a,16 b closest the nearest output points 18, 19, α usually represents anattenuation that can be realized by a simple circuit. Thus, therespective output signals Y of the amplifiers 35, 37 are madeindependent of each other. As a result the feedback amplifiers 35, 37are forced to operate as separate feedback loops and the counter noiseintroduced at the output points serves to make Va−αVb zero, i.e. asignal indicative of the local noise except for the counter-noiseintroduced by the nearby output point 19.

Although the embodiment of FIG. 4 has been shown with a single thefeedback has been described in terms of single end gain factoradjustment circuits 40, 42 coupled to specific inputs of amplifiers 35,37, it will be understood that desensitizing of the inputs can in factbe realized by using gain factor adjustment at other inputs, or at bothinputs. Also signals derived by weighing noise on more than two inputpoints on power supply conductor 10 may be used. As an alternative, theoutputs of the amplifiers 35, 37 may be arranged so that they generatesubstantially no input signal at a nearby input of another amplifier.This may be realized for example by using two outputs coupled to powersupply conductor 10 with output signals of opposite sign, and anamplitude ratio, so that no net output signal results at the nearbyinput.

Although the invention has been illustrated for one power supplyconductor 10, it will be understood that similar feedback arrangementsmay be applied to both power supply conductors 10, 11. The power supplyconductors 10, 11 may be of linear shape, or be an essentially onedimensional structure running along a path with bends in it. In bothcases noise can be suppressed effectively with a pair of amplifiers 35,37. However, without deviating from the spirit of the present invention,power supply conductors 10, 11 may extend in two dimensions. In thiscase, however, more than two non-local feedback circuits are preferablyused, with inputs and outputs distributed over a two dimensional region,always with measures to counteract that coupling between nearby inputsand outputs lead to localized noise compensation. Furthermore, theinvention may of course be applied to noise media other than powersupply lines, for example to semi-conductor substrates underneathsub-circuits 12 a-d.

1. An integrated circuit comprising a noise source, a sub-circuit, anoise medium capable of transferring noise signals from the noise sourceto the sub-circuit, a feedback circuit, having an input coupled to thenoise medium at an input point on a first side of the sub-circuit and anoutput coupled to the noise medium at an output point on a second sideof the sub-circuit, the first and second sides being opposite to oneanother relative to the sub-circuit, the noise source being coupled tothe noise medium on said second side, comprising a further feedbackcircuit, with input and output coupled to the noise medium, so that anoutput of the feedback circuit is coupled to the noise medium closer tothe input of the farther feedback circuit than to the input of thefeedback circuit, the outout of the feedback circuit and the input ofthe further feedback circuit being mutually arranged so that undesiredfeedback of output signals from the output of the feedback circuit bythe further feedback circuit is at least partially suppressed.
 2. Anintegrated circuit according to claim 1, wherein said noise medium is apower supply line.
 3. An integrated circuit according to claim 1,wherein the further feedback circuit has differential inputs and meansfor altering an amplitude of an input signal at at least one of thedifferent inputs such that mutually different weights are applied tosignals at the different inputs, the mutually different weights being sothat said undesired feedback is at least partially suppressed.
 4. Anintegrated circuit composing a noise source, a circuit, a noise mediumcapable of transferring noise signals from the noise source to thesub-circuit, a feedback circuit, having an input coupled to the noisemedium at an input point on a first side of the sub-circuit and anoutput coupled to the noise medium at an output point on a second sideof the sub-circuit, the first and second sides being opposite to oneanother relative to the sub-circuit, the noise source being coupled tothe noise medium on said second side, comprising a further noise sourcecoupled to the noise medium and a further feedback circuit with inputand output coupled to the noise medium, the inputs of the feedbackcircuit and the further feedback circuit being differential inputs, thedifferential inputs of the feedback circuit being coupled to the noisemedium on mutually opposite sides of the further noise source, thedifferential inputs of the further feedback circuit being coupled to thenoise medium on mutually opposite sides of the noise source.